A flat sandwich-type cell filled with liquid crystals (LC) is the main element of electrooptical devices. One or another electrooptical effect is displayed in the flat cells depending on an initial orientation of the LC molecules. A way of distribution of the LC molecules inside the cell over the surface of the support substrates depends, for the first, on the properties of mesophase itself (nematic, cholesteric, or smectic) and, for the second, on the orienting properties of the surface of the support substrate. As a rule, the initial orientation of the LC molecules is a parallel (planar) one or a perpendicular (homeotropic) one. In the first case, axis of the LC molecules are parallel to the support substrate, and in the second case they are perpendicular to the support substrate.
Thin alignment layers for which different polymer materials are used (polyamides, polyimides, polyvinyl alcohol, polyesterketones, siloxanes, etc.) have been applied on the surfaces of the support substrates to render the orienting properties thereto.
The Russian Patent No. RU 2,082,209 of Jun. 20, 1997 discloses the use of polyvinyl alcohol films as an alignment layer (orientant) in the LC cells designed to prevent blinding during operation with bright light sources.
The Russian Patent No. RU 2,017,186 of Jul. 30, 1994 discloses the use of a polyimide film as an orientant in the LC displays.
The technology of LC cells production includes a high-temperature step of sealing the cells. Their operation is often performed in the aggressive atmosphere and high humidity conditions. Therefore, the use of the widely known polymer materials as orientants in some cases is restricted by their low chemical and thermal resistance, hydrolytic stability.
Polyimide films which are known by their high thermal resistance have been recently used as orientants. As a rule, a film of polyamidoacid is firstly applied from a solution onto a substrate by the centrifugation method, and then its imidization is performed at a high temperature, for example, at 300° C. (see the U.S. Pat. No. 4,781,439 of Nov. 1, 1988 and U.S. Pat. No. 6,639,646 of Oct. 28, 2003).
The U.S. Pat. No. 6,094,251 of Jul. 25, 2000 discloses the use of polyimides and polyamides (“Probimide 32” by Ciba-Geigy company and polymers SE130, SE7311, SE4110 and SE610 by Nissan company) as the orientants. In order to change the alignment properties of the applied orientants, their thermal treatment was performed. One layer was heated at a temperature of less than 180° C. within half an hour, and the other at a temperature of about 300° C. for 1-3 hours.
In order to render new properties to the polymer orientants, their chemical modification is also possible. For example, a polyimide film is exposed to radicals of fluorocarbons in a high-frequency plasma with a power of 30-300 mW/cm2 for 10-100 seconds, thereby a controlled increase in pre-inclination angles of the liquid crystal director is achieved (see the Russian Patent No. RU 2,055,384 of Feb. 27, 1996).
When a polymer alignment layer is formed from a solution, a necessary step is its solidification or drying, which significantly extends the duration of technological cycle for the manufacture of electrooptical devices.
The polymer orientants are applied onto the surface of the support substrates from solutions by different methods such as centrifugation, dipping, spraying by an atomizer, coating through a slotted nozzle. The technology of electrooptical device manufacture makes quite strict demands on the alignment layer formation method: high continuity, uniformity, and possibility to adjust the thickness on the surfaces of great dimensions. The known alignment layer formation methods have some essential disadvantages: the centrifugation method can not be applied to deposit uniform coatings on the plates of great dimensions due to significant difference in linear velocities of different areas of the plate; the spraying by an atomizer does not allow to obtain high uniformity in thin layers, requires a significant material consumption, and is uneconomical; the coating through a slotted nozzle (spinneret) does not allow to obtain uniform thin films.
As a rule, polymer layers applied onto the support substrates from the solutions do not exhibit a sufficient orienting effect in relation to the LC molecules. There are different methods to render the orienting properties to the surface of the polymer.
A method of mechanical texturing (rubbing) of the surface is widely used. The essence of this method is in rubbing of the film surface in one direction by means of a roller covered by a pile cloth having short polymer fibres (for example, viscose, polyesters, nylon, etc.). An example of the mechanical method for rendering the orienting properties to a polyimide film is disclosed in the U.S. Pat. No. 6,219,123 of Apr. 17, 2001.
The mechanical method by rubbing of aligning films has some essential disadvantages: generation of dust particles on the surface of the films; appearance, on the surface of the films, of static electric charges which discharge may result in damage of thin-film transistors; impossibility to keep strictly the predetermined direction of fibres, which leads to non-reproducibility of the obtained results and, therefore, to high percentage of faulty production; incompatibility of this method with the conditions of clean rooms in which the process operations to assemble the LC devices (indicators, displays, etc.) are performed.
In case of the polymers produced by vapor deposition onto the substrates, for example, by polymerization in plasma, a structure which is inherently an orientant for the LC molecules is generated at once during the polymer layer formation process. Thus, there is no need in any process step of mechanical rubbing.
In the U.S. Pat. No. 4,038,439 of Jul. 26, 1977, in order to obtain the perpendicular orientation of LC molecules, polished glass substrates are kept in a flow of free fluorocarbon radicals that are generated by decomposition, in plasma, of polyfluorinated compounds, preferably tetrafluoroethylene. A thickness of the polyfluorinated orientants is 100 to 500 Å, and a deposition rate is 2 to 5 Å/min.
The Russian Patent No. RU 2,073,902 of Feb. 20, 1997 proposes a method for unidirectional orientation of liquid crystals by means of the surface of substrates by deposition of a plasma-polymerized material, wherein the support substrates of LC cells are placed inclined in the plasma formation region with respect to electrodes of the diode system, and a glow-discharge plasma containing hydrocarbon radicals is generated.
It is known that the plasma polymerization processes do not allow us to produce polymer films of a stable and predetermined chemical composition. As the result, it is difficult to get a reproducible homogeneous orientation of the LC molecules, what is the main disadvantage of the formation of orientant layers by the plasma polymerization method. Moreover, the produced layers have poor mechanical properties and can be destroyed in the period of electrooptical devices operation.
Known is a method for producing films and coatings of substituted and unsubstituted poly-para-xylylenes by pyrolysis of a cyclic di-para-xylylene ([2.2]-para-cyclophane) or a corresponding substituted derivative thereof (The Encyclopedia of polymers, Moscow, “Soviet Encyclopedia” Publisher, 1974, vol. 2, page 871). The cyclic di-para-xylylene is subjected to pyrolysis at 600° C. in vacuum having a pressure of less than 133.322 n/m2 (less than 1 mm of mercury). Vapors of the para-xylylene generated thereby react by polyrecombination during deposition of said vapors on any surface to produce a film, wherein the polymer yield is a quantity one. This method can be used to deposit the poly-para-xylylene films (with a thickness of from 20 nm to 250 μm) on articles of various cross-sections. For this purpose, the articles are placed in a condensation chamber which is directly connected to a pyrolitic tube.
The U.S. Pat. No. 5,177,475 of Jan. 5, 1993 proposes to use poly-para-xylylene as an alignment layer for LC devices, wherein poly-para-xylylene simultaneously acts as a blocking layer for the direct current (DC) in such devices. In the specification to this patent, poly-para-xylylene is referred to as Parylene, i.e. a trade name for a family of polymers (poly-para-xylylenes) developed by Union Carbide Corporation, USA. The main representative of such family is an unsubstituted poly-para-xylylene which has a trade name of Parylene N and which is a linear highly-crystalline material. As the consequence of unique nature of the vapor deposition process, the parylene polymers can be produced as structurally-continuous films having a thickness of from fractions of a micron up to tens of microns. It is mentioned in the specification to this patent that the Parylene film production process is very well known, and that is why this process is nowhere disclosed therein. According to this patent, it is preferable to use the unsubstituted poly-para-xylylene (Parylene N) as a polymer orientant, though it is also possible to use other Parylenes.
There are various well-known methods (see the U.S. Pat. No. 3,830,733 of Aug. 20, 1974; the USSR Author's Certificate No. SU 1,151,546 of Apr. 23, 1985) for production of films and coatings from poly-para-xylylene or substituted derivatives thereof by sequential execution, in vacuum in a three-zone reactor which consists of a sublimation zone, a pyrolysis zone, and a condensation zone, of sublimation, pyrolysis of a cyclic di-para-xylylene or substituted derivatives thereof and condensation of the pyrolysis products on a substrate simultaneously with their polymerization.
However, these known methods do not allow to fabricate poly-para-xylylene films possessing the orienting properties with respect to the LC molecules in a single process step. For example, it is stated in the Russian Patent No. RU 2,215,770 of Nov. 10, 2003 that a poly-para-xylylene film used for encapsulation of the LC molecules does not provide a controllable orienting action.
A prior art closest to the clamed invention is a method (see the U.S. Pat. No. 3,342,754 of Sep. 19, 1967) for production of linear homopolymers of poly-para-xylylene or substituted derivatives thereof by sequential execution, in vacuum in a three-zone reactor consisting of a sublimation zone, a pyrolysis zone, and a condensation zone, of sublimation, pyrolysis of a cyclic di-para-xylylene or substituted derivatives thereof and condensation of the pyrolysis products on a substrate simultaneously with their polymerization to produce a polymer layer.
According to this method, the pyrolysis step is performed at a temperature between 450 and 700° C., and the condensation and simultaneous polymerization are performed at a temperature of less than 200° C., depending on the used cyclic di-para-xylylene ([2.2]-para-cyclophane); a residual pressure in the system is kept in the interval between 0.0001 and 10 mm of mercury; and a partial pressure of vapors of p-xylylene biradicals is kept below 0.75 mm of mercury. A molecular of the cyclic di-para-xylylene according to this method can contain up to 6 substituent groups in aromatic rings, wherein the substituent groups can be as follows: various hydrocarbons, oxyhydrocarbons, thiohydrocarbons, hydroxyls, halogens, nitro groups, nitrile groups, amino groups, and mercapto groups. Produced by this method is a linear, non-cross-linked, thermoplastic polymer product completely soluble in solvents without molecular destruction processes being occurred, and the melting of its crystalline phase is in a narrow temperature range. The polymer product is obtained as a homogeneous coating on the substrate surface.